168 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



This generalization is accepted in other countries, thus in 

 Norway the Kongsberg silver lodes are only valuable where they 

 traverse the fahlbands, and in Australia the auriferous quartz 

 lodes of the Grympie district are only of value while passing 

 through the black slate. 



This relation of mineral contents to enclosing rock is even 

 observable on a microscopic scale, thus Dr. Wadsworth says 

 "It is not uncommon to find minute veins in rocks, which under 

 the microscope shew variations in their filling material as they 

 pass through different minerals.* The bearing of these observa- 

 tions on the theory of lateral secretion will be considered here- 

 after. 



4. — Lodes of tin or copper in killas dipping towards or into 

 a great mass of granite or elvan are usually richer than similar 

 lodes dipping away from such masses. The Dolcoath main lode 

 and the lode at Wheal Eliza are good examples of the value of 

 this character. 



5. — Lodes, and especially the rich parts of lodes, are usually 

 bounded by rocks exhibiting notable alteration phenomena, in 

 other words, valuable metalliferous deposits are rarely if ever 

 found in highly crystalline rocks, whether stratified or eruptive, 

 which present little or no evidence of secondary change. The 

 changes are, in some instances, merely alterations of structure, 

 but in most cases there are also extensive chemical alterations. 



The most notable structural alteration is the development 

 of a tabular or "sheeted" structure in the country rock by 

 means of a series of joint-planes parallel to the walls of the 

 vein. Similar planes are sometimes also observable in the vein 

 substance. This sheeted structure must not be confounded with 

 foliation, lamination, quarry cleavage, or slaty cleavage (strain- 

 slip-cleavage), all of which aid in the division of rock masses 

 into tabular fragments, that is into fragments having two 

 dimensions much greater than the third. These are due to 

 causes which are essentially independent of the metalliferous 

 deposits with which they are associated, although their pre- 

 existence may have acted favourably by facilitating the produc- 



*Proc. Boston Soc. Nat. His., May, 1884. 



